It has been known to use metallocene-catalyzed polyethylenes, especially “mLLDPE” to make films. Such films offer toughness that is superior to other LLDPEs. This and other advantages of mLLDPE films is discussed by D. M. Fiscus and P. Brandt in “Oriented Exceed™ Metallocene LLDPE Films, A New Option in Oriented Films” in TECHNICAL ASSOCIATION OF PAPER AND PULP INDUSTRY (TAPPI) POLYMERS, LAMINATIONS AND COATINGS CONFERENCE, August 1998; and by the same authors in “Film Property Enhancement by Orienting Metallocene LLDPE” in SCHOTLAND CONFERENCE, Houston, Tex. September 1997 and NEW PLASTICS '98 CONFERENCE in London, England, January 1998. Such films also offer improved optics as well as low levels of extractables and blocking resistance. These improvements have allowed converters to down gauge the films, thus lowering the cost. However, processing these mLLDPE is somewhat challenging due to the relatively low viscosity due in part to the narrow molecular weight distribution of the mLLDPEs. Forming biaxially oriented films—those possessing transverse direction orientation (“TDO”) and machine direction orientation (“MDO”)—from mLLDPEs is thus challenging.
There has been at least one disclosure of the blending of two or more LLDPEs having metallocene-like characteristics, such as in U.S. Ser. No. 11/726,806. However, the blend disclosed in that case was such that the second mLLDPE made up less than 10 wt % of the composition of the film layer. Thus, an improvement in processability is not seen. Other relevant disclosures include WO 2007/130277, WO 2004/031291 and WO 2004/022634 that discuss the use of one or more LDPEs and/or LLDPEs for films. There is still a desire to improve the processability of LLDPEs having metallocene-like properties, while maintaining their advantageous properties.
The inventor has found that processability can be improved for mLLDPEs (or LLDPEs that possess certain properties similar to those of many mLLDPEs) by either blending at least two mLLDPE of different density and/or melt index, or by blending in a short chain branched LLDPE having improved processability. It is believed that the blends, or the short chain branched LLDPE alone, possess somewhat broader molecular weight distributions and thus improved processability is achieved without losing the desirable toughness of mLLDPEs.